1. Field of the Invention
The invention has as its object a process for the hydrogenation of nitrobenzene with hydrogen to hydrazobenzene at temperatures ranging from 40.degree. to 110.degree. C. in the presence of a precious metal catalyst and preferably of a co-catalyst, and in the presence of an aqueous alkaline solution and of an organic solvent, the reaction mixture being maintained in turbulent motion, wherein o-, m- or p-chloronitrobenzene is used as nitrobenzene, the reaction is carried out in an organic solvent in which the dichlorohydrazobenzene is readily soluble when warm and difficultly soluble when cold, and that the precious metal catalyst is employed in such amount that the weight ratio of nitrobenzene to precious metal catalyst used is in the range 1:0.00005-less than 0.0002, and preferably between 1:0.00005 to less than 0.0001.
2. Discussion of the Prior Art
It is known to prepare hydrazobenzenes, starting from the nitro stage, by reduction with zinc and alkali. (Alexeyev, Zeitschrift fur Chemie, 1868, 497). Working with zinc, which entails many drawbacks, may be avoided by the use of other reducing agents. However, the reducing agents used, such as dextrose (U.S. Pat. No. 2,794,047), formaldehyde (U.S. Pat. No. 2,794,046), hydrazine hydrate (German patent application DAS No. 2,609,530), sodium sulfide (German patent application DOS No. 2,546,656) or sodium hydrogen sulfide (Belgian Pat. No. 832,268), iron and acid (German patent application DOS No. 2,535,045), and sodium amalgam (German Pat. No. 1,668,898), are difficult to handle technically and, in addition, usually require specially produced azoxybenzenes as starting materials.
Processes for the production of 2,2' or 3,3'-dichlorohydrazobenzene, respectively, are described in U.S. Pat. No. 3,205,217, German patent application DAS No. 2,609,530 and U.S. Pat. No. 3,156,724, for example. In these prior-art processes, the reduction is carried out with hydrogen and a catalyst. While these hydrogenation processes are easier to handle than the processes described above, they have a number of serious drawbacks. In U.S. Pat. No. 3,205,217, for example, a cobalt-cyanide complex is proposed as catalyst. Working with cyanides calls for extensive safety measures, especially in operating on the industrial scale. A further disadvantage is that this process results in a mixture of different reaction products, which complicates any subsequent purification operations.
In the process described in German patent application DAS No. 2,609,530 (examples 4 and 5), specially produced azoxybenzenes are used. This two-stage procedure makes the process complicated. Another drawback is that relatively large amounts of Raney nickel are required as catalyst.
U.S. Pat. No. 3,156,724 describes a process for the preparation of 2,2'-dichlorohydrazobenzene in which o-chloronitrobenzene is hydrogenated with hydrogen at elevated pressure and at temperatures ranging from 40.degree. to 100.degree. C. in the presence of a palladium or platinum catalyst and of a naphthaquinone as cocatalyst in an aqueous alkaline medium, and preferably in the presence of a solvent which dissolves both the starting product and the end product. Proposed solvents are aromatic hydrocarbons such as toluene, xylene or benzene. The main drawback of this prior-art process is that complex purification operations are required, as pointed out above, to separate the 2,2'-dichlorohydrazobenzene from the undesired byproducts. Even so, only moderately pure 2,2'-dichlorohydrazobenzene is obtained as distillation residue whose melting point differs by a few degrees from that of the pure substance. Another drawback is that relatively large amounts of the catalyst combination are required. (The weight ratio of cocatalyst to o-chloronitrobenzene is to be in the 0.004:1 to 0.008:1 range. The preferred weight ratio of precious-metal catalyst to o-chloronitrobenzene is given as 0.0002:1 to 0.001:1.)
Of the hydrazobenzenes, the dichlorohydrazobenzenes especially are of economic importance. By comparison with unsubstituted hydrazobenzene, they are far less of a health hazard and therefore safer to handle. Of the dichlorohydrazo compounds, 2,2'-dichlorohydrazobenzene particularly is of growing interest as an intermediate product in the manufacture of pigments, e.g., 3,3'-dichlorobenzidine, as described in U.S. Pat. No. 4,075,198. However, 2,2'-dichlorohydrazobenzene is of economic importance also as a starting product in the manufacture of pharmaceutical products.
Dichlorohydrazobenzenes must meet very high purity requirements particularly when they are to be used as intermediate or primary products in the manufacture of pigments since the byproducts, and especially substances formed through over- or underreduction, will interfere with the benzidine or semidine rearrangement which follows and therefore will adversely affect the quality of the pigment. The purification methods employed up to now to separate the by-products formed during the reaction have not been satisfactory in practice. The reason is that it has been necessary to treat the reaction product after separation of the catalyst repeatedly with aqueous mineral acids for extraction of the strongly basic by-products such as aniline and to remove in subsequent further washing operations the excess mineral acid and the salts formed. Despite these repeated washing operations, a purity better than 98 percent usually cannot be achieved. Moreover, there is the risk that an undesirable rearrangement may occur.
It is the object of the present invention to provide a process which commercially is easy to handle, is not afflicted with the drawbacks described above, and furnishes the desired hydrazobenzenes in good quality, especially insofar as purity is concerned, and in good yields.
This object is accomplished by proceeding as specified below.